(i) Field of the Invention
The present invention relates to an ink composition favorably used for inkjet recording; an inkjet recording method; and a printed material using the same. Further, the present invention relates to a planographic printing plate obtained by using the ink composition and a method of producing the planographic printing plate. Specifically, the present invention relates to an ink composition suitable for inkjet recording, which can be cured with high sensitivity by irradiation of a radiation ray to form a high quality image, and is superior in storage stability and continuous ejection stability; an inkjet recording method; a printed material using the same; a planographic printing plate obtained by using the ink composition; and a method of producing the planographic printing plate.
(ii) Description of the Related Art
Image recording methods for forming an image based on image data signals on a recording medium such as paper include methods such as an electrophotographic method, a sublimation or fusion heat-transfer method, and an inkjet method. Among these, the inkjet method can be carried out using an inexpensive device, wherein the image is directly formed by ejecting ink only onto a desired image region on the recording medium, thus enabling an efficient use of ink and resulting in low running costs. In addition, the inkjet method generates little noise and is superior as an image recording method.
The inkjet method can be applied for printing not only on plain paper but also on a non-water absorptive recording medium such as a plastic sheet or a metal plate. However, higher speed and higher image quality at the time of printing are important issues, and the time period needed for drying and hardening ink droplets after ejection has a great influence on the printing efficiency and the quality of a printed image.
One of such inkjet recording methods is a method using an inkjet recording ink which is curable by irradiation with a radiation ray. In this method, printing efficiency can be improved and a sharp image can be obtained, by curing droplets by irradiating with a radiation ray immediately, or a given period of time, after the ejection.
By achieving higher sensitivity of inkjet recording ink which is curable by irradiation with a radiation ray such as an ultraviolet ray, the ink can be imparted with higher hardening properties, and thus many benefits can be obtained, such as improved inkjet recording efficiency, reduced power consumption, longer lifetime due to a reduced load on a radiation ray generator, and prevention of evaporation of low-molecular substances due to insufficient hardening. In addition, improvement in sensitivity is effective in increasing the strength of the image formed with the inkjet recording ink and, in particular, in the case of preparation of planographic printing plates, it may increase the hardness of the image region and printing durability.
Such inkjet methods using an ink composition which is curable by a radiation ray such as ultraviolet light have attracted attention recently for being relatively odorless, quick in drying, and capable of recording on a recording medium having reduced ink absorbency. Ultraviolet light curable ink compositions for inkjet recording utilizing radical polymerization have been disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 63-235382, 3-216379, and 5-214280, and Japanese Patent Publication (JP-B) Nos. 6-21256 and 6-62905). In addition, compositions containing polymerizable compounds of specific radically polymerizable acrylates and a colorant have been proposed (see, for example, JP-A Nos. 2003-192943 and 2003-192944), for the purpose of providing an inkjet recording ink that can record an image having higher sensitivity and adhesiveness without ink bleeding, even on a recording medium on which it is normally difficult to record directly using the inkjet recording method, the ink being yet higher in stability with less skin irritation or sensitization. While such radically polymerizable inks are superior in hardening speed and can form an image without ink bleeding, they have had a disadvantage in that the adhesiveness to a recording medium deteriorates due to volume shrinkage during hardening.
Accordingly, for the purpose of improving the adhesiveness to a recording medium, cationically polymerizable ink compositions resistant to shrinkage during ultraviolet hardening have been proposed (see, for example, JP-A No. 9-183928). However, these cationically polymerizable inks have insufficient stability during storage because of the reaction of acids generated therein over time, which is a significant obstacle to commercialization of such inks. For improvement of the storage stability, methods of adding a basic compound or a thermal base-generating agent have been proposed (see, for example, JP-A Nos. 2003-312121, 2003-341217 and 2004-91558). However, a new problem has emerged in that the hardening sensitivity of the ink is lowered due to the basic compound inhibiting the function of the acid generated by light exposure. In particular, in the case of inkjet ink, there is a demand for satisfying both stability and high sensitivity, since the stability inside the head has a significant influence on ejecting properties.
Further, in the case of cationically polymerizable ink, there is a demand for preventing the acid in a polymerizing catalyst from remaining and thus causing degradation of an image or a substrate, even after formation of the image.